diff --git a/package.json b/package.json
index 84cda2c..f775b28 100644
--- a/package.json
+++ b/package.json
@@ -70,7 +70,8 @@
     "lint-staged": "^15.0.0",
     "prettier": "^3.0.0",
     "typescript": "^5.0.0",
-    "typescript-eslint": "^8.18.2"
+    "typescript-eslint": "^8.18.2",
+    "@eslint/js": "^9.30.0"
   },
   "husky": {
     "hooks": {
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index 2df5e77..384faf1 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -75,6 +75,9 @@ importers:
         specifier: ^3.23.8
         version: 3.25.67
     devDependencies:
+      '@eslint/js':
+        specifier: ^9.30.0
+        version: 9.30.0
       '@types/debug':
         specifier: ^4.1.12
         version: 4.1.12
diff --git a/src/hooks/useDownloader.ts b/src/hooks/useDownloader.ts
index 811da03..754a6e1 100644
--- a/src/hooks/useDownloader.ts
+++ b/src/hooks/useDownloader.ts
@@ -207,8 +207,14 @@ export function useDownloader(uploaderPeerID: string): {
         console.error('[Downloader] no stream found for', message.fileName)
         return
       }
-      setBytesDownloaded((bd) => bd + (message.bytes as ArrayBuffer).byteLength)
+      const chunkSize = (message.bytes as ArrayBuffer).byteLength
+      setBytesDownloaded((bd) => bd + chunkSize)
       fileStream.enqueue(new Uint8Array(message.bytes as ArrayBuffer))
+      dataConnection.send({
+        type: MessageType.Ack,
+        fileName: message.fileName,
+        offset: message.offset + chunkSize,
+      } as z.infer<typeof Message>)
       if (message.final) {
         console.log('[Downloader] finished receiving', message.fileName)
         fileStream.close()
diff --git a/src/hooks/useUploaderConnections.ts b/src/hooks/useUploaderConnections.ts
index f23d7ed..15be3ad 100644
--- a/src/hooks/useUploaderConnections.ts
+++ b/src/hooks/useUploaderConnections.ts
@@ -9,8 +9,28 @@ import { decodeMessage, Message, MessageType } from '../messages'
 import { getFileName } from '../fs'
 import { setRotating } from './useRotatingSpinner'
 
-// TODO(@kern): Test for better values
-const MAX_CHUNK_SIZE = 256 * 1024 // 256 KB
+const MIN_CHUNK_SIZE = 128 * 1024 // 128 KB
+const MAX_CHUNK_SIZE_CAP = 1024 * 1024 // 1 MB
+
+function calculateChunkSize(totalTransferSize: number): number {
+  const MIN_TRANSFER = 1 * 1024 * 1024 // 1 MB
+  const MAX_TRANSFER = 1 * 1024 * 1024 * 1024 // 1 GB
+
+  if (totalTransferSize <= MIN_TRANSFER) {
+    return MIN_CHUNK_SIZE
+  }
+
+  if (totalTransferSize >= MAX_TRANSFER) {
+    return MAX_CHUNK_SIZE_CAP
+  }
+
+  const ratio =
+    (totalTransferSize - MIN_TRANSFER) / (MAX_TRANSFER - MIN_TRANSFER)
+
+  return (
+    MIN_CHUNK_SIZE + Math.round(ratio * (MAX_CHUNK_SIZE_CAP - MIN_CHUNK_SIZE))
+  )
+}
 
 function validateOffset(
   files: UploadedFile[],
@@ -31,6 +51,8 @@ export function useUploaderConnections(
   files: UploadedFile[],
   password: string,
 ): Array<UploaderConnection> {
+  const totalTransferSize = files.reduce((sum, f) => sum + f.size, 0)
+  const MAX_CHUNK_SIZE = calculateChunkSize(totalTransferSize)
   const [connections, setConnections] = useState<Array<UploaderConnection>>([])
 
   useEffect(() => {
@@ -237,33 +259,10 @@ export function useUploaderConnections(
                     final,
                   }
                   conn.send(request)
-
-                  updateConnection((draft) => {
-                    offset = end
-                    if (final) {
-                      console.log(
-                        '[UploaderConnections] completed file',
-                        fileName,
-                        '- file',
-                        draft.completedFiles + 1,
-                        'of',
-                        draft.totalFiles,
-                      )
-                      return {
-                        ...draft,
-                        status: UploaderConnectionStatus.Ready,
-                        completedFiles: draft.completedFiles + 1,
-                        currentFileProgress: 0,
-                      }
-                    } else {
-                      sendNextChunkAsync()
-                      return {
-                        ...draft,
-                        uploadingOffset: end,
-                        currentFileProgress: end / file.size,
-                      }
-                    }
-                  })
+                  offset = end
+                  if (!final) {
+                    sendNextChunkAsync()
+                  }
                 }, 0)
               }
 
@@ -309,6 +308,36 @@ export function useUploaderConnections(
               break
             }
 
+            case MessageType.Ack: {
+              const { fileName: ackFileName, offset: ackOffset } = message
+              try {
+                const ackFile = validateOffset(files, ackFileName, ackOffset)
+                updateConnection((draft) => {
+                  if (draft.uploadingFileName !== ackFileName) {
+                    return draft
+                  }
+
+                  const completed = ackOffset >= ackFile.size
+                  return {
+                    ...draft,
+                    uploadingOffset: ackOffset,
+                    currentFileProgress: Math.min(ackOffset / ackFile.size, 1),
+                    ...(completed
+                      ? {
+                          status: UploaderConnectionStatus.Ready,
+                          completedFiles: draft.completedFiles + 1,
+                          uploadingFileName: undefined,
+                        }
+                      : {}),
+                  }
+                })
+              } catch (err) {
+                console.error('[UploaderConnections] error handling ack:', err)
+              }
+
+              break
+            }
+
             case MessageType.Done: {
               console.log(
                 '[UploaderConnections] transfer completed successfully',
diff --git a/src/messages.ts b/src/messages.ts
index a9acf2b..3072c6a 100644
--- a/src/messages.ts
+++ b/src/messages.ts
@@ -6,6 +6,7 @@ export enum MessageType {
   Start = 'Start',
   Chunk = 'Chunk',
   Pause = 'Pause',
+  Ack = 'Ack',
   Done = 'Done',
   Error = 'Error',
   PasswordRequired = 'PasswordRequired',
@@ -48,6 +49,12 @@ export const ChunkMessage = z.object({
   final: z.boolean(),
 })
 
+export const AckMessage = z.object({
+  type: z.literal(MessageType.Ack),
+  fileName: z.string(),
+  offset: z.number(),
+})
+
 export const DoneMessage = z.object({
   type: z.literal(MessageType.Done),
 })
@@ -80,6 +87,7 @@ export const Message = z.discriminatedUnion('type', [
   InfoMessage,
   StartMessage,
   ChunkMessage,
+  AckMessage,
   DoneMessage,
   ErrorMessage,
   PasswordRequiredMessage,